The field of this invention concerns the determination of food freshness, and, in particular, methods and devices for the detection of microbial spoilage in food products.
The spoilage and souring of perishable foods with time is an on-going problem for the consumer and food product provider alike. Although some deterioration in freshness is due to oxidative processes, spoilage and souring is in large part due to the growth of microbes such as bacteria, yeasts, and fungi. To derive energy for their growth, these microbes break down food carbohydrates, proteins and fats. The breakdown process produces a variety of low molecular weight molecules such as carboxylic acids (e.g., lactic and acetic acids), aldehydes, nitrogen containing molecules including ammonia, trimethylamine, urea and small diamines, and sulfur compounds. For example, over time, microbes in milk and dairy products produce an increased amount of lactic acid and lactic acid derivatives resulting in sour and odorous milk, respectively. (See, e.g., Gyosheva, B. H., (1982) xe2x80x9cCompounds forming the aroma complex of Bulgarian sour milkxe2x80x9d Milchwissenschaft 37, 267-289).
At present, there are no inexpensive, simple and accurate measurement devices for detecting food spoilage at a consumer level. Spoilage has conventionally been monitored by standard bacteriological and chemical laboratory methods, while certain more esoteric assays have been discussed in the literature to approve speed or cost of detection. For example, electrochemical assays involving gamma-irradiation immobilization of lactate oxidase in poly (vinyl alcohol) on platinized graphite electrodes have been proposed for lactate detection in dairy products (See Hajizadeh, K., et al. (1991) xe2x80x9cImmobilization of lactate oxidase in a poly(vinyl alcohol) matrix on plantinized graphite electrodes by chemical cross-linking with isocyanatexe2x80x9d Talanta 38, 37-47). Other approaches to food spoilage monitoring include, for example, using non-membrane disposable oxygen electrode systems for detection of milk spoilage from aerobic bacteria (See Bell, C. Ackland, et al. (1995) xe2x80x9cDisposable oxygen electrode system without membranes applied to the detection of ultrahigh-temperature milk spoilagexe2x80x9d Netherlands Milk and Dairy Journal 49, 139-149); and modified Orion ammonia electrodes for the trimethylamine detection in fish (See Chang, G. W., et al. (1976) xe2x80x9cTrimethylamine-specific electrode for fish quality controlxe2x80x9d Journal of Food Science 41 723-724). However, these techniques are not practical at a consumer level and smell, color or taste has been the major way consumers detect spoilage.
Recently, there have been some attempts at finding a way to help consumers determine whether foods are contaminated by specific toxins. For example, U.S. Patent No. 5,306,466 by Goldsmith discloses packaging with a bar code design formed of labeled antibodies bound to toxins. The bar code design is placed in contact with the juices of a food product, for example by printing it on a membrane in the packaging itself, and a competitive antibody-antigen reaction is used to detect spoilage. The antibodies on the membrane, which react with the specific class of toxins accumulating in the food product""s juices, are released from the membrane, thereby destroying the design and providing a visual indication of toxin presence. However, this type of indicator is expensive because of the antibody cost, may not be safe, and has limited applicability.
Similarly, electrode systems, and electrochemical or competitive assay techniques, do not solve the consumer problem. These techniques often involve relatively lengthy or complex procedures, and may have limited applicability. Accordingly, there exists a need for relatively rapid and efficient, accurate, inexpensive and simple methods and devices for indicating microbial spoilage in a variety of food products. Such methods and devices would simplify spoilage indication techniques while maximizing their accuracy, efficacy and applicability through selection of indicators and barriers.
Accordingly, an object of the invention is to provide a method of indicating food spoilage which is simple, accurate and inexpensive.
Another object of the invention is to provide a food spoilage indicator device for placing in fluid contact with a food product.
These and other objects and features of the invention will be apparent from the following description and the drawings.
The present invention features methods and devices for providing a detectable, e.g., visual, indication of microbial food spoilage. Briefly, a barrier which allows passage of a reactant molecule of a predetermined size but segregates out larger molecules, separates the food product from a carrier which carries an indicator. The indicator provides a detectable change, such as a color change or fluorescence, upon reaction with the reactant molecule. The present invention is also based on the further recognition that such an indicator, carrier and barrier can be placed in a food product package without contaminating the food.
As used herein, the following terms are to be understood in light of the following definitions:
The term xe2x80x9cfluidxe2x80x9d refers to liquid or gaseous states;
The term xe2x80x9cbarrierxe2x80x9d means a device or portion of a device which provides a physical separation between one location and another or between the device and the product;
The term xe2x80x9creactant moleculexe2x80x9d means any molecule produced by microbes in a food product causing spoilage, or a product of a secondary reaction of a molecule produced by the microbes in the food product causing spoilage, which can react with the selected indicator in the device of the invention;
The term xe2x80x9cindicatorxe2x80x9d means any material which can react with the reactant molecule to produce a detectable, e.g., visual change, either through a direct or intermediate reaction;
The term xe2x80x9cmicrobexe2x80x9d means any bacteria, yeast, viruses, fungi, and any similar organism that can cause food spoilage;
The term xe2x80x9cspoilagexe2x80x9d means any change in a food product making it less palatable or dangerous for consumption by the animal, e.g., human, that would normally eat the food product; and
The term xe2x80x9ccarrierxe2x80x9d means any material which entraps or holds an indicator; e.g., paper, other fibrous or cellulosic materials and the like.
The invention concerns a method for detecting microbial spoilage in a food product. A spoilage indicator device is placed in fluid contact with a food product. Reactant molecules produced in the food product by microbial spoilage are allowed to traverse a barrier separating the food product from the carrier in the device and react with the indicator material in the carrier. The detectable change caused by the reaction of the reactant molecule with the indicator indicates a build-up of the reactant molecule in the food product and, therefore, microbial spoilage. The barrier sheet should be permeable to molecules under about 200 daltons but substantially impermeable to larger molecules.
The reactant molecules are produced directly by the microbes or are intermediate by-products related to microbial growth in a food product undergoing spoilage. These reactant molecules exist in liquid and/or gaseous forms and are normally acids, bases, aldehydes, sulfur compounds, or their derivatives, depending upon the type of microbes causing the microbial spoilage. In certain preferred embodiments, the selected barrier will only allow passage of reactant molecules which are in a non-ionized form.
The indicator material is selected based upon the type of food product, the type of suspected microbes, and the type of reactant molecule which is expected to be detected. Preferably, the indicator material produces a visual detectable change, such as a chromatic or fluorescent change, in response to pH or other changes caused by a reaction between the reactant molecules and the indicator materials. Preferred indicator materials are capable of producing detectable changes with pH changes. Preferred pH indicators have a detectable change within the range of about 3.0 to about 8.4. Each individual indicator material produces a detectable change in response to a narrower pH range and the particular indicator used is selected by food type and reactant molecule expected. Preferably, the indicator materials are sensitive enough to produce detectable changes when the reactant molecules are in a concentration of about 0.01% by volume in the fluid surrounding the food product. Additional indicators preferentially react with sulfur containing groups, aldehydes, or nitrogen containing groups such as ammonia, urea or amines. While these materials may cause pH changes as well, sulphydryl or nitrogen compound specific indicators are known in the art. Specific examples of indicator materials involving chromatic changes include, but are not limited to, Bromophenol blue, Bromocresol green, Methyl red, Litmus, Bromocresol purple, Bromothymol blue, Phenol red, Thymol blue, Schiffs base reagent, diphenyl, dinitrophenylhydrazine and sodium nitroferricyanide. Indicator materials involving fluorescent detectable changes include, but are not limited to, Dichlorofluoroscein, Calcein, and Fluorescein.
The choice of carrier and barrier sheet generally depend upon the reactant molecules being detected as well as the chosen indicator material. The carrier must be capable of entrapping the indicator but must allow the reactant molecule access to the indicator. Porous, inert materials are preferred for use as carriers. The carrier generally has a thickness of less than about 1 mm. Preferred carrier materials include, but are not limited to, papers (e.g., untreated cellulose), polyamides, cellulose acetate, gels, foams, glass fibers, plastics, and resins such as an ion-exchange resins. Suitable barrier materials must have the proper molecular weight cut-off properties, be chemically inert, and non-contaminating to the food product. Preferred barrier materials include, but are not limited to, polyethylenes, polyvinyl chlorides or other water resistant or hydrophobic materials. Typically, the barrier sheet has a thickness in a range of about 1 xcexcm to about 13 xcexcm.
The spoilage indicator device can also include an outer layer which is transparent, translucent and/or has at least a lucent or transparent portion through which an underlying layer, such as the carrier, can be observed. Such an outer layer can be disposed directly on a second surface of the carrier or on a barrier sheet wrapped around the carrier""s first and second surfaces. In some instances, the barrier sheet material can act as the outer layer. Preferred outer layer materials include, but are not limited to, cellulose acetate, vinyl polymers (such as PVC), polyethylene, polypropylene, polystyrene, polycarbonate, polyester thermoplastic, glass or polyamide film.
In a preferred embodiment of the invention, the food spoilage indicator device includes Phenol red A as the indicator entrapped within a paper carrier for detecting an amine produced by spoilage of the food product. In this embodiment, the carrier""s first surface is separated from fluid contact with the food product by a barrier such as a food wrap layer, e.g., polyvinyl chloride or polyethylene.
In another preferred embodiment of the invention, the food spoilage indicator device includes a Litmus or a Bromocresol purple indicator material entrapped within a paper carrier for the detection of an acid such as a lactic acid. In this embodiment, the carrier is separated from fluid contact with the food product by a barrier which is acid stable, e.g., hydrophobic layer which allows passage of the reactant molecule while restricting the flow of the liquids of the food product. In a most preferred embodiment, this spoilage indicator device also includes a xe2x80x9cwindowxe2x80x9d or a transparent or translucent layer which allows visualization of the indicator from outside the packaging. This window could be a cellulose acetate, glass, polycarbonate, polystyrene or polypropylene outer layer disposed on a second surface of the carrier.
Other features and aspects of the invention will be apparent from the detailed description and the drawing.